ASUS STRIX Z490-F GAMING - No POST, stubborn board

[DynaxSC]

Hi,

I have this very stubborn ASUS mobo on the bench, and I already face the wall with it. It just refuses to POST, although everything seems to be OK.

The situation looks like this:

After connecting the board to the power supply on Nuvoton (SLP_S3#, pin 64), the Low signal (50 ohm resistance) appears immediately. PWRNTN# signal goes through Nuvoton to the chipset (confirmed after removing the chipset). The following conditions are then met for the chipset to generate the SLP_S3# signal:

1. RTC oscillates normally (32.768KHz).
2. RTCRST # is at H level.
3. RSMRST # is H level
4. PWRBTN # arrives at the chipset (checke after taking of the chipset)
5. LOW_BAT # (BATLOW #) is inactive, H level

The chipset and Nuvoton have all the necessary supply powers.

I read somewhere that all these parameters above must be met for the chipset to generate SLP_S3#. The problem, however, is that the chipset immediately after turning on the 5VSB activates SLP_S3#, before the PowerButton is pressed and the board is ready to run the normal power on sequence process. Hence the Nuvoton SIO (NCT6798D) does not generate PSON# signal at all.

An additional curiosity is that after turning on the board, there is no +5V_DUAL voltage, all the mosfets switching this voltage, controlled by the UP7501 chip (specialized logic chip for driving 5V_DUAL), do not turn on, i.e. they do not transfer voltage from 5VSB to 5V_DUAL in stand-by mode. I checked if the UP7501 chip gets all the conditions to turn on 5V_DUAL, and they are all met. After replacing the UP7501, nothing has changed, so probably something else drags the mosfet-key control signal to L level (the UP7501 output is Open Drain).

After removing the chipset, SLP_S3# is not pulled down to ground any more, so generally it must be the chipset that pulls this signal to zero for some other reason.

I also replaced the chipset itself with a 100% working one, but nothing has changed, so it's most probably not a faulty chipset.

There is also no reading of the BIOS, because in general the CPU VRM has no power at all (by not turning on 5V_DUAL), but I do not know if this is a normal situation on this board, usually CPU VRMs have a solid stand-by power, but this board has a mosfet switch turning it on, so maybe it's normal. Both BIOS chips have been removed / checked and are OK.

I also read in the specification of the Z490 chipset that if the THERMTRIP# signal from the processor is active, it may stop the chipset to start the board (with SLP_S3#) - unfortunately I did not check that when the chipset was removed. An active THERMTRIP# signal does an immediate and unconditional power-off of the board by the chipset.

The question is whether someone has any idea what it could be, because something is putting the chipset into an incorrect state right after turning on the stand-by power (5VSB). The board has no visible mechanical damage or visible burned parts, broken tracks or missing parts.

I also attach a schematic of a very similar board, many same part labels are to be found on my board, nethertheless this schematics seems to be slightly simpler than my board. Unfortunately there is no schematic or boardview of my particular board available anywhere.

Thanks in advance for your help.

MOD Edit: Schematic moved to new thread - https://www.badcaps.net/forum/showthread.php?t=124067
.

[anikin73]

Re: ASUS STRIX Z490-F GAMING - No POST, stubborn board

first you manage to power on,so slp_s3 did appear. your issue is cpu voltage did not appear. You should check whether CVCC_SA and VCC_IO appear as these 2 will lead to the enabling of the pwm chip of the cpu mosfets.

[DynaxSC]

Re: ASUS STRIX Z490-F GAMING - No POST, stubborn board

That was also my initial idea, but then I traced back the EN signal for the mosfet keys which give the power to the CPU VRM chip (and 5V_DUAL), and these signal is forced down to zero by something, I cannot locate on this board. So despite the UP7501 gives H signal (open drain output) to eneble power to the CPU VRM, something else connected in parallel to this EN signal is preventing this signal to go H.

Another aspect is that SLP_S3 is immediatelly active after powering the mobo (only with +5VSB present on ATX). And this is not a normal situation. SLP_S3# schuold go active after Power Button is pressed, or if the board was powered off active, and the power applied within ca 4 seconds. In any other case, SLP_S3# must be H (inactive) after applying only +5BSB.

I also checked the THERMTRIP signal, and there is no voltage on it, but the signal is High impedance, so seems it is in an undefined state, which is not relevant in the state in which the chipset is (probably no external pull-up resistor present, but it is probably inside the chipset, but the chipset does not pull this signal up in the state in which it is when +5VSB is enabled).

So this is very weired what this board is doing.

[anikin73]

Re: ASUS STRIX Z490-F GAMING - No POST, stubborn board

in 2nd and 3rd gen board, slp_s3 appear when you power on. I think your problem should be why the cpu VRM did not work or the voltage sequences before it did not work.
If you insist on slp_s3, it means your PCH is gone and can throw away....

[DynaxSC]

Re: ASUS STRIX Z490-F GAMING - No POST, stubborn board

Hi, you are right with SLP_S3#, I was somehow stuck in my thinking like this board . It must be low in stand-by mode, and it should goes H on Power Button press. I have to rethink now, thanks for being persuasive to my point of view. The issue still persists, but at least you unblocked my thinking .